In cases in which rough irregularities arise on the molding surfaces of a mold employed to mold metals and resins, such irregularities are transferred to the surface of a molded article (referred to below as “workpiece”). Thus, finishing of the surface of the workpiece is required after molding. In addition, because the surface of the workpiece engages with the irregularities on the surface of the mold, reducing the demoldability, productivity during molding decreases significantly. Moreover, there is a concern regarding deformation and damage to the workpiece due to the need to apply a strong force when demolding, and the rate of defects also increases.
Thus, the surfaces of a mold are normally finished smooth by hand polishing. This not only enables a smooth finish to be achieved on surfaces of the workpiece, but also secures demoldability.
However, along with molds having increasingly complex shapes, there is now a demand for shorter lead times for mold deliveries. The polishing of surfaces of molds by hand, which is both labor intensive and time consuming, is an impediment to meeting such demands, and causes an increase in mold fabrication costs.
Moreover, depending on the shape and material of the workpieces to be molded, the required demoldability is not always obtainable even when mold surface are polished to smooth surfaces.
Thus, various methods have been proposed to improve the demoldability of workpieces. For example, there is a proposal to increase the draft angle provided to cavities of molds, and there is moreover a proposal to perform surface treatment to enhance the slipperiness of surfaces of molds, e.g. by forming a fluorine based coating or a diamond-like carbon (DLC) film.
Furthermore, in contrast to making surfaces of molds smooth surfaces, there is also a proposal to form irregularities of a predetermined shape thereon. An example of this is a proposal for a “method of treating the surface of a cavity of a die used for casting” to improve fluidity while maintaining good release properties. In this proposal, spherical ejection particles of 100 to 1000 μm that have a hardness at least as hard as a casting mold are ejected against cavity surfaces of a casting mold to form semi-spherical dimples thereon (see claim 1 and claim 2 of Patent Document 1).